918 GEOLOGY OF OHIO. 
boundary, and balancing the many faulty regions within the field against 
the known extensions of the seam beyond the limits taken, we find the 
areas of the coal, 5 feet and upwards in thickness, to make an aggregate 
of 94,156.8 acres or 147.12 square miles.* Some reduction from these 
figures will be required in accounting for the coal already mined in 
the Valley, but the facts bearing upon this important question will be 
better appreciated after the survey of the several subdivisions of the 
field, and this topic will accordingly be taken up in a subsequent part of 
this chapter. 
In addition to the coal of the great seam, there are several other 
sources of coal supply within the district. The Lower Kittanning coal 
is occasionally mined on'a small scale, as is also the gLower Freeport 
seam, while the Upper Freeport coal here becomes the basis of large 
mining operations. It is the only seam of the three that makes a really 
important contribution to the coal resources of this region, and there 
are large areas in which it would, of itself, furnish a proper basis for 
extensive work. 
General Section of the Field. 
The geological range of the Hocking Valley field is shown in the 
appended section. It will be seen from this that it extends from the 
Mercer horizon to the Crinoidal or Ames limestone. In the high ridge 
of Trimble township, between Snow Fork and Sunday Creek, the 
strata rise at least 100 feet above the Crinoidal limestone, or nearly to 
the place of the Pittsburgh coal, but these upper strata include no 
valuable or easily recognized elements, and they occupy a comparatively 
small area, and need not therefore be counted in the geological column. 
The Crinoidal limestone is reached in only a'small territory, and very 
rarely in the same hills that cover the Middle Kittanning coal. 
The column is as follows: 
Crinoidal limestone—Ames, of Andrews ...........scecsssescssscscsseeee 5 feet. 
Interval, mainly shales, red and drab, carrying nodules of 
hematite at one or MOre HOriZONS...............02.eceeeesescecseeee 45 “ 
Ewing limestone, nodular and uncertain ................cescseeeeeeees \ DR 
Often replaced by iron OTe ...............eseeseeee 
Intervalmainly,shalesi 2. wiv. csecceadssessceccoeaceseetesseeeer se eecee 40 “ 
Cambridge limestone, frequently doubled .....................+200+ \ 2-10 « 
_ (Sometimes bears iron ore, “‘black limestone’’) ......... 
*The method used in determining the acreage consisted in cutting out an accurate map of the field 
including outliers, in weighing the map, and in computing its area by comparison with the weight 
of a standard area. 
